Because rendering on the GPU isn't a panacea. On mobile platforms, it can have negative implications on battery life and performance, and given the sequential-d

Because rendering on the GPU isn't a panacea. On mobile platforms, it can have negative implications on battery life and performance, and given the sequential-dependent nature of CSS rendering, I'm not sure you'd even get the benefit of parallelization. In many cases, you'd end up waiting for the main thread most of the time anyway (which you'd probably also get if you just spawned a few threads on another CPU core). It also adds a hell of a lot of complexity since SOCs have varying levels of capability in their GPUs, which your code now needs to identify, integrate and test against. I'm not claiming there's no benefit, but there's a reason they don't just GPU all the things.